Stereochemistry Exercise 4

In the previous videos on stereochemistry you stated when you have a dash in the 4th order group such as hydrogen you assume the rotation is the opposite after you order the other groups, so R becomes S because a clockwise changes to counter clockwise. In this video you did bond rotations by swapping out three of the bonds. My question is why did you have to do that in these compounds, but did not for the dashed hydrogen stereocenters in the previous video?

I did the rotation here because the #4 priority group was in the plane of the page. It’s a choice – one could either do a bond rotation, or swap two groups and then remember to “flip” the R/S designation one gets. Rotation seemed to me to be the easiest thing to visualize here. I hope it was clear.